1. Field of the Invention
The present invention relates to an improved printing method for tinting synthetic resin sheets used in the manufacture of laminated safety glass. More particularly, the invention relates to an improved method for obtaining a gradient color band on the synthetic resin sheet component of laminated safety glass used in automobile windshields.
2. Description of the Prior Art
Laminated safety glass is well knwon in the art for use as glazing units for vehicles, especially automobile windshields, and for architectural uses. Laminated safety glass comprises at least one shield of glass laminated to at least one sheet of a synthetic resin material which is usually referred to as the interlayer. The laminate may be the sandwich type construction wherein the interlayer is interposed between two pieces of glass. Alternately, the laminate may comprise a single sheet of glass laminated to a synthetic resin sheet (also referred to as an interlayer for the purposes of the present invention). In the latter bilayer type of laminated safety glass the synthetic resin sheet may optionally be coated with or laminated to another synthetic polymeric material.
Inks for tinting interlayer material used in the preparation of laminated safety glass are well known in the prior art. These inks are described, inter alia, in U.S. Pat. Nos. 2,636,420, 2,739,080, 2,914,373, 3,346,526, 3,441,361, and 3,591,406. These inks, which comprise a dye and a solvent, are normally applied to the interlayer by contact printing methods which are well known to those skilled in the art. The preferred method is rotogravure printing which method involves the printing of a series of dots in a gradient pattern using a rotogravure type roll. The printed interlayer is then positioned so that the gradient band appears at the top of the windshield in the resulting laminate.
The inks of the prior art used in the printing of plastic sheets for laminated safety glass suffer from one or more deficiencies such as low dye concentration (i.e. less than 10% by weight of dye based on the total weight of dye and solvent), narrow operating temperature range, tendency of the freshly printed sheet to smear, volatility, flammability and toxicity. The low concentration of dye in the solvents of the prior art requires the use of large cells on the rotogravure printing cylinder in order to hold enough ink to obtain the desired density in the darker part of the gradient. These large cells may give rise to printing defects such as mottle, that may require seasoning, i.e., heating of the printed sheet, in order to cause diffusion of the dyes. Small cell spacing i.e., 102 to 178 microns (4 to 7 mils) would be expected to give more uniform printing but they may not provide the optical density required for the darker portions of the gradient bands found in windshields. Attempts to obtain larger cell volume with small cell spacing by making deeper cells leads to cell wall collapse and further printing defects that require seasoning. A need exists in the art for an ink system with a high concentration of dyes in order to obtain the desired optical density.
The deficiencies in the processes of the prior art used to print a gradient band are overcome by the present invention, which provides an ink system having a high concentration of dyes in a solvent with a low order of volatility, flammability and toxicity.